The present invention relates generally to Code Division Multiple Access (CDMA) systems, and more particularly, to radio resource control signaling for HSDPA in WCDMA systems.
High Speed Downlink Packet Access (HSDPA) is a packet data service offered in Wideband Code Division Multiple Access (WCDMA) networks. The HSDPA is an evolution of WCDMA specified by the Third Generation Partnership Project (3GPP) in Release 99 of the WCDMA standard. The HSDPA, introduced in Release 5 of the WCDMA standard, provides peak data rates up to 10 Mbits/s using enhanced features such as higher-order modulation (16 QAM), physical layer retransmission with soft combining, hybrid automatic repeat request (H-ARQ), multicode transmission, fast link adaptation, and fast scheduling. The transport channel for HSDPA is the High Speed Downlink Shared Channel (HS-DSCH). The HS-DSCH is carried over the High Speed Physical Downlink Shared Channel (HS-PDSCH).
The HS-DSCH is a time multiplexed channel shared by a plurality of mobile stations. Mobile stations are scheduled to receive data on the HS-PDSCH by a serving base station. The scheduling interval is referred to as a Transmission Time Interval (TTI). During a given TTI, one or more mobile stations may be scheduled. The mobile stations report channel conditions to the base station on an uplink channel called the High Speed Dedicated Physical Control Channel (HS-DPCCH) to enable the base station to make scheduling decisions. The base station schedules the mobile station based, at least in part, on the reported channel conditions. The identity of the mobile stations scheduled to receive packet data on the HS-DSCH in a given TTI is transmitted on the High Speed Shared Control Channel (HS-SCCH). The HS-SCCH is also used to send transmission parameters needed by the mobile station to decode the HS-DSCH, such as the code channels, the transport block size, and the modulation scheme used in the corresponding TTI.
In release 5 of the WCDMA standard, the HS-DSCH always operates in conjunction with an Associated Dedicated Physical Channel (A-DPCH). The A-DPCH carries power control signals and Radio Resource Control (RRC) messages between the mobile station and the base station. Radio resource control is a protocol that provides control of the mobile station by a radio network controller in a radio access network. Release 6 of the WCDMA standard allows RRC signaling messages to be transmitted to the mobile station over the HS-PDSCH instead of the A-DPCH. The reason for using in band signaling on the HS-PDSCH is to reduce the amount of power allocated for signaling purposes so that more power can be allocated to the HS-DSCH. In release 6 of the WCDMA standard, RRC messages may be sent in band on the HS-PDSCH. When in band signaling over the HS-PDSCH is used, the DPCH is used to carry only power control information and is called a fractional-DPCH (F-DPCH).
Unlike the A-DPCH, the HS-PDSCH does not use soft handover and transmission of RRC signaling to the mobile station is likely to be less reliable than using the A-DPCH. Channel conditions may fluctuate rapidly on the HS-PDSCH, particularly when the mobile station is moving at a high rate of speed. If critical RRC messages, such as handover messages, are lost due to poor channel conditions, radio link failure may be the result. Therefore, there is a need to make signaling between the base station and the mobile station more robust, particularly when in-band signaling on the HS-PDSCH is used to carry RRC messages.